Catalytic burner

ABSTRACT

A catalytic burner includes a porous diffuser member and a porous distributor member. At least one seal extends between the porous diffuser member and the porous distributor member thereby defining an oxidation chamber. Catalyst media particles for catalyzing an oxidation reaction of a fuel stream to produce heat and an exhaust stream is disposed within the oxidation chamber. The catalyst media particles comprise carrier particles. The carrier particles comprise refractory material, and at least some of the carrier particles have thereon an outer coating comprising an oxidation catalyst. The catalyst media particles have a void fraction of at least 0.6.

STATEMENT OF FEDERALLY-SPONSORED RESEARCH

This invention was made with Government support under Contract No.DE-EE0003491, awarded by the Department of Energy. The Government hascertain rights in this invention.

TECHNICAL FIELD

The present disclosure relates to catalytic burners and catalytic mediaused in them.

BACKGROUND

In commercial boilers, gaseous hydrocarbon fuel sources are converted tothermal energy inside a heat exchanger contained within the boiler.Typically, the space inside the heat exchanger is kept small to minimizethe size of the boiler. To minimize operating and environmental cost, itis desirable that the fuel is efficiently converted to thermal energyand the amount of deleterious byproducts (e.g., NO_(x), CO) isminimized.

Previous attempts to address this problem were directed to controllingthe fluid dynamics of the gas-air mixture through perforated distributorand diffuser plates (this term of art is commonly used even though the“plates” may not be flat), which are commonly oriented as concentric,coaxial cylinders and constructed of a heat-resistant metal such asstainless steel. While these designs are compact and reduced the NO_(x)emissions, the reductions are becoming insufficient to meet increasinglystringent emissions standards.

SUMMARY

The present disclosure overcomes the foregoing problems by creating ahigh void fraction packed catalyst media particle bed disposed betweenthe distributor and diffuser plates. The catalyst media particles lowerthe effective combustion temperature, and thus reduces the amount ofthermal NO_(x) generated. A packed catalyst media particle bed isdesirable to achieve adequate residence time of the fuel in the presenceof the catalyst. However, the thickness (or depth) and void fraction (ε)of the catalyst media particle bed greatly affect the rate of heattransfer of the combusted fuel in the catalyst media particle bed. For acatalyst media particle bed that is too thick and/or has too low of avoid fraction, the heat from the combustion reaction is ineffectivelyremoved from the catalyst media particle bed, causing the temperature ofthe bed to increase rapidly and potentially exceed the stability limittemperature of the catalyst, thus making it inactive.

Depending on the thickness of the catalyst media particle bed, at lowvoid fractions such as in the case of spherical catalyst media particles(e.g., monodisperse spherical particles having a void fraction of about0.36) or crushed particles the bed temperature may exceed 800° C.

As used herein, the term “void fraction” refers to the ratio of thevolume of void space between particles to the total container volume(V_(container), expressed in cubic centimeters (cm³)) in a containerpacked full with the particles and is defined by the followingexpression:

$\begin{matrix}\frac{V_{container} - V_{exterior}}{V_{container}} & (1)\end{matrix}$

where V_(exterior) is the exterior volume in cubic centimeters (cm³),including all solid material, open pores, and impervious portions. Theexterior volume is defined as

V_(exterior)=W−S  (2)

where W is the saturated weight in grams, S is the suspended weight ingrams, and the test liquid is water having a density of 1 gram per cubiccentimeter (g/cm³). These terms can be calculated, for example,according to the procedure described for burned refractory brick in ASTMDesignation C20-00 (Reapproved 2010), “Standard Test Methods forApparent Porosity, Water Absorption, Apparent Specific Gravity, and BulkDensity of Burned Refractory Brick and Shapes by Boiling Water” fromASTM International, West Conshohocken, Pa.

High bed temperatures can cause the catalyst to become deactivatedand/or volatilize off the surface and be carried away from the systemwith the exhaust stream. While the catalyst media particles can achievelow NO_(x) emissions, the high catalyst bed temperatures progressivelyreduce the effectiveness of the catalyst.

At greater void fractions the bed temperature falls with increasing voidfraction, accompanied by increasing catalyst media particles life, untilat void fractions greater than 0.6, the bed temperature typically doesnot exceed 800° C., and the palladium catalyst exhibits long life whilestill achieving low NO_(x) emissions.

In some embodiments, the catalyst media particles are substantially freeof, or even free of, internal void space.

Accordingly, in one aspect, the present disclosure provides a catalyticburner comprising:

-   -   a porous diffuser member;    -   a porous distributor member;    -   at least one seal extending between the porous diffuser member        and the porous distributor member, thereby defining an oxidation        chamber; and    -   catalyst media particles for catalyzing an oxidation reaction of        a fuel stream to produce heat and an exhaust stream, wherein the        catalyst media particles is disposed within the oxidation        chamber, wherein the catalyst media particles comprise carrier        particles, each carrier particle comprising refractory material,        wherein at least some of the carrier particles have thereon an        outer coating comprising an oxidation catalyst, and wherein the        catalyst media particles have a void fraction of at least 0.6.

In some embodiments, the porous diffuser member and the porousdistributor member comprise substantially parallel porous plates. Insome embodiments, the porous diffuser member comprises a first open end,the porous distributor member is disposed at least partially inside theporous diffuser member, and the porous distributor member comprises asecond open end.

Features and advantages of the present disclosure will be furtherunderstood upon consideration of the detailed description as well as theappended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic perspective view of an exemplary catalytic burner100 according to the present disclosure.

FIG. 1A is an enlarged detail of region 1A in FIG. 1.

FIG. 2 is a schematic cross-sectional view of exemplary catalytic burner200.

FIG. 3 is an enlarged schematic cross-sectional view of an exemplarycatalyst media particle 270.

FIG. 4 is a schematic cross-sectional view of exemplary catalytic burner400.

FIG. 5 is a photomicrograph of an exemplary saddle-shaped carrierparticle prepared in the PREPARATION OF CATALYST MEDIA PARTICLES E.

FIG. 6 is a schematic cross-sectional view of an exemplary boilerincluding a catalytic burner according to the present disclosure.

Repeated use of reference characters in the specification and drawingsis intended to represent the same or analogous features or elements ofthe disclosure. It should be understood that numerous othermodifications and embodiments can be devised by those skilled in theart, which fall within the scope and spirit of the principles of thedisclosure. The figures may not be drawn to scale.

DETAILED DESCRIPTION

Referring to FIG. 1, exemplary catalytic burner 100 has inlet opening110, adjacent seal 160, for a fuel stream (not shown) that passesthrough holes 113 (see FIG. 1A) in porous distributor member 115, isoxidized internally within the burner, and then released as an exhauststream (not shown) through holes 129 in porous diffuser member 120.

Components of the catalytic burner, other than the catalyst mediaparticles, may comprise any material capable of withstanding the heat(e.g., up to at least 800° C.) and corrosion due the oxidation of thefuel stream. Stainless steels (e.g., in grade 304 or 316) are exemplarysuch materials.

The catalytic burner may have any suitable design, for example, as knownin the art, including ones not exemplified herein. Examples of variousdesigns include cylindrical (e.g., as shown in FIGS. 1-3), frustoconical(e.g., as described in PCT Publ. Appl. No. WO 2011/076220 A1 (Möller)),parallel plate, and spheroidal configurations (e.g., as described inU.S. Pat. No. 5,474,443 (Viessmann et al.))

The porous distributor member typically comprises a wall with aplurality of holes extending therethrough may be shaped, for example, asa cylindrical tube, conical tube, hollow spheroid, or a combinationthereof, although this is not a requirement. The holes may be of anyshape (e.g., slits and/or round holes) and pattern, but are preferablysufficiently sized that the catalyst media particles do not pass throughthem. In some embodiments, the plurality of holes in the wall of theporous distributor member comprises from 1 to 30 area percent,preferably from 5 to 25 area percent, and more preferably 5 to 15 areapercent, based on the total surface area of the wall, although this isnot a requirement. Preferably, the holes should be sufficiently smallthat flash back ignition is prevented. The selection of hole shape andsize may depend on the composition of the fuel stream and its pressure,and will be known to those of ordinary skill in the art. In someembodiments, the porous distributor member comprises a sintered porousmetal frit.

Likewise, the porous diffuser member typically comprises a wall with aplurality of holes extending therethrough may be shaped, for example, asa cylindrical tube, conical tube, hollow spheroid, or a combinationthereof, although this is not a requirement. As before, the holes may beof any shape and pattern, but are preferably sufficiently sized that thecatalyst media particles do not pass through them. In some embodiments,the plurality of holes in the wall of the porous distributor membercomprises from 10 to 90 area percent, preferably from 15 to 45 arepercent, based on the total surface area of the wall, although this isnot a requirement. In some embodiments, the porous diffuser membercomprises a wire mesh.

Typically, the porous diffuser member is adapted to at least partiallyenclose the porous distributor member while creating an oxidationchamber (e.g., at least partially filled with catalyst media particles)that has a substantially constant gap between the porous diffuser memberand the porous distributor member. For use in residential boilers thegap is typically from 0.5 millimeter to 3 millimeters, while higher gapsare typical with larger burners intended for use with commercial andindustrial boilers.

In some embodiments, the porous distributor member and/or the porousdiffuser member is/are hollow.

In one embodiment, shown in FIG. 2, catalytic burner 200, having thesame exterior general appearance of catalytic burner 100 (shown in FIG.1), comprises porous diffuser member 220, which comprises a cylindricalconduit 221 having a wall 222 with a plurality of holes (not shown)therethrough, and which comprises first and second open ends 225, 227.Porous distributor member 230, which comprises a cylindrical conduit 231having a wall 232 with a plurality of holes 233 therethrough, isdisposed inside porous diffuser member 220 and comprises third andfourth open ends 235, 237. First open end 225 is covered by first endcap 240 and comprises an alignment pin 245 inwardly extending alonglongitudinal axis 250 of porous diffuser member 220. Second end cap 244covers fourth open end 237, except for central opening 239 adapted toreceive alignment pin 245. Annular seal 260 extends between porousdiffuser member 220 and porous distributor member 230, thereby defining(in combination with first end cap 240) oxidation chamber 270.

Catalyst media particles 280 are disposed within oxidation chamber 270,and catalyze an oxidation reaction in the fuel stream to produce heatand an exhaust stream. Catalyst media particles 280 comprise carrierparticles 284 that comprise refractory material. The catalyst mediaparticles pack with a void fraction of at least 0.6.

Optionally a tubular screen may be disposed inside the porousdistributor member to prevent any fragments of catalyst media particlesthat may form from passing inwardly through holes in porous diffusermember.

Referring now to FIG. 3, carrier particle 184 has an outer coating 186comprising an oxidation catalyst 182. Outer coating 186 comprises innerlayer 188 comprising cerium oxide in contact with carrier particle 184and supports outer layer 189 comprising the oxidation catalyst.

In another embodiment, shown in FIG. 4, cylindrically-shaped catalyticburner 400, having the same exterior general appearance of catalyticburner 100 (shown in FIG. 1), comprises porous diffuser member 420,which comprises a cylindrical conduit 421 having a wall 422 with aplurality of holes (not shown) therethrough, and which comprises firstand second open ends 425, 427. Cylindrically-shaped porous distributormember 430, which comprises a cylindrical conduit 431 having a wall 432with a plurality of holes 433 therethrough, is disposed inside porousdiffuser member 420 and comprises third and fourth open ends 435, 437.First open end 425 is covered by end cap 440. Third open end 435 engagesgroove 446 in end cap 440 thereby forming a seal. Annular seal 460extends between porous diffuser member 420 and porous distributor member430, thereby defining (in combination with end cap 440) oxidationchamber 470. Catalyst media particles 480 are disposed within oxidationchamber 470.

In use, the catalyst media particles catalyze an oxidation reaction offuel stream to produce heat and an exhaust stream. The catalyst mediaparticles comprise a plurality of carrier particles, at least some ofwhich have an outer coating on at least a portion of their respectiveouter surface. The carrier particles are typically intentionally shaped(i.e., not randomly shaped as, for example, with crushed particles orpowders) due to the process used to make them (e.g., by extruding ormolding), although this is not a requirement. The selection ofdimensions of the catalyst media particles is typically controlled, atleast in part, by the size of the burner and the spacing between theporous distributor member and the porous diffuser member. In someembodiments, the catalyst media particles have an average particlediameter (i.e., the largest dimension of the particle) or largest of 0.5to 20 millimeters, 0.5 to 6 millimeters, 0.5 to 3 millimeters, althoughother particle diameters can also be used. Since the outer coating istypically relatively thin, the shape and size of the catalyst mediaparticles is typically substantially the same as that of the carrierparticles.

The average spacing between the diffuser member and the distributormember is generally larger than the average particle diameter of thecatalyst media particles. In some embodiments, the average spacingbetween the diffuser member and the distributor member is from 0.5 to 20millimeters, from 0.5 to 10 millimeters, or from 1 to 6 millimeters,although this is not a requirement.

In some embodiments, the shape of the carrier particles ispredetermined, for example, such as would result from a molding process.The carrier particles comprise, and may be composed of refractorymaterial (i.e., one or more refractory materials). Examples of suitablerefractory materials include aluminas (e.g., alpha alumina, betaalumina, gamma alumina, eta alumina, and/or theta alumina), mullite,aluminum titanate, zirconia, zircon, silica, fireclay (an impurekaolinite), cordierite, silicon carbide, and mixtures thereof.Additional examples of suitable refractory materials include oxides ofmetals selected from the group consisting of magnesium, aluminum,titanium, vanadium, chromium, manganese, iron, cobalt, nickel, cerium,copper, zinc, gallium, germanium, strontium, yttrium, zirconium,niobium, molybdenum, ruthenium, rhodium, silver, samarium, indium, iron,tin, antimony, barium, lanthanum, hafnium, tungsten, rhenium, iridium,platinum, and combinations thereof. Preferred refractory materialscomprise inorganic metal oxides (e.g., aluminas and mullite).Preferably, the refractory material has relatively low thermalcoefficient of expansion and/or are capable of enduring many thermalcycles without fracture and fragmentation.

Useful carrier particles may be obtained from commercial sources orprepared, for example, according to known methods. For example, aluminacarrier particles can be made by extruding and optionally shaping (e.g.,curling) a slurry or sol-gel comprising an alumina precursor (e.g.,colloidal boehmite), cutting the extrudate to the desired length, dryingthe extrudate particles, optionally calcining them, and then sinteringthem generally as described in U.S. Pat. No. 4,314,827 (Leitheiser etal.); U.S. Pat. No. 4,518,397 (Leitheiser et al.); U.S. Pat. No.4,881,951 (Monroe et al.). Suitable saddle-shaped carrier particles canbe prepared by extruding a boehmite sol-gel through a hollow needle witha bent tip as described in the PREPARATION OF CATALYST MEDIA PARTICLES Ein the examples section hereinbelow, and shown in FIG. 5.

In some embodiments, the carrier particles have a substantially constantcross-sectional profile along their length. This is especially the casefor carrier particles that are formed by an extrusion process.

The catalyst media particles pack with substantial space between them(e.g., they pack with a void fraction of at least 0.6). In someembodiments, the void fraction is at least 0.63, at least 0.65, at least0.68, at least 0.70, at least 0.72, at least 0.75, at least 0.78, oreven at least 0.80. Preferably, the catalyst media particles do not packin an ordered array.

Any shape or combination of shapes that packs with the desired voidfraction can be used. Examples of shaped particles that may pack withthe desired void fraction include bent rods, bent tubes, andsaddle-shaped particles (e.g., as in the examples hereinbelow).

The catalyst media particles may have any suitable theoretical density.For example, they may have an theoretical density of at least 1.5 g/cm³,at least 1.7 g/cm³, at least 1.9 g/cm³, at least 2.1 g/cm³, at least 2.3g/cm³, at least 2.5 g/cm³, at least 2.7 g/cm³, at least 2.9 g/cm³, atleast 3.0 g/cm³, at least 3.1 g/cm³, at least 3.3 g/cm³, at least 3.5g/cm³, at least 3.6 g/cm³, at least 3.7 g/cm³, at least 3.8 g/cm³, oreven at least 3.9 g/cm³.

At least some of the carrier particles, preferably at least a majorityor even all of the carrier particles, have an outer coating on at leasta portion of their outer surface. The degree of coverage by the outercoating will typically depend on the process used to deposit it on thecarrier particles. For example, a solvent coating process may beeffective to coat the entire surface, while physical vapor depositionmay result in partial coverage of the surface of individual carrierparticles. The outer coating may comprise one or more layers (e.g., one,two, or three layers). The outer layer may have any thickness, but istypically relatively thin. For example, it may have a thickness of from1 nanometer (nm) to 10 microns, preferably from 1 nanometer to 1 micron.

The outer coating comprises an oxidation catalyst for oxidizing theoxidizable gas in the fuel stream. Such materials are well known in theart and include, for example, materials comprising platinum, iridium,osmium, palladium, ruthenium, rhodium, alloys thereof, oxides thereof,and combinations thereof. Of these palladium and its oxides arepreferred.

Palladium-based catalysts are known to possess high catalytic activityfor methane oxidation. The most active phases in these catalysts arebelieved to be in the oxide form which is typically stable between 300to 600° C. At elevated temperatures above about 800° C., in addition toa decrease in active surface area through, e.g., sintering, palladiumoxide (PdO) decomposes to Pd metal which is believed to be less activethan the oxide form. Importantly, the metallic form also has a highervolatility than the oxide, and therefore poses a greater risk forcatalyst material loss through vaporization processes during burneroperation. These considerations indicate that an optimal temperaturerange between 300 to 600° C. results in efficient and long-term use ofpalladium catalysts in catalytic burners according to the presentdisclosure.

In some embodiments, the outer coating comprises an inner layer and anouter layer, which layers may comprise different materials. In oneembodiment, the inner layer (i.e., the layer in contact with the carrierparticle) comprises cerium oxide. The inner layer supports the outerlayer comprising the oxidation catalyst. An exemplary two-layerconstruction and its preparation are described in the PREPARATION OFCATALYST MEDIA PARTICLES G in the examples hereinbelow.

In some exemplary embodiments, catalytically-active palladium isdeposited onto the carrier particles using physical vapor deposition.Physical vapor deposition refers to the physical transfer of palladiumfrom a palladium-containing source or target to the carrier particles.Physical vapor deposition may be viewed as involving atom-by-atomdeposition, although in actual practice, the palladium may betransferred as extremely fine bodies constituting more than one atom perbody. Once at the surface of the carrier particles, the palladium mayinteract with the surface physically, chemically, ionically, and/orotherwise.

There are different approaches for carrying out physical vapordeposition (PVD). Representative approaches include sputter deposition,evaporation, and cathodic arc deposition. Any of these or other PVDapproaches may be used, although the nature of the PVD technique usedcan impact catalytic activity. For instance, the energy of the physicalvapor deposition technique used can impact the mobility, and hencetendency of the deposited palladium atoms and clusters to agglomerateinto larger bodies on the surface of the support. Higher energy tends tocorrespond to an increased tendency of the palladium to agglomerate.Increased agglomeration, in turn, tends to reduce catalytic activity.Generally, the energy of the depositing species is lowest forevaporation, higher for sputter deposition (which may include some ioncontent in which a small fraction of the impinging metal species areionized), and highest for cathodic arc (which may be several tens ofpercents of ion content). Accordingly, if a particular PVD techniqueyields deposited palladium that is more mobile than might be desired, itmay be useful to use a PVD technique of lesser energy instead.

Physical vapor deposition generally is a line of sight/surface coatingtechnique between the palladium source and the carrier particles. Thismeans that only the exposed, outer surfaces of the carrier particles,but not inner pores (if any) that are well within the substrate, aredirectly coated. While the inner surfaces not in a direct line of sightwith the source will also tend not to be directly coated with palladium,on some substrates, the atoms and small clusters of the depositedpalladium can penetrate by diffusion a small distance into porouscarrier particles.

In some preferred embodiments, the active palladium species is collectedessentially completely on the outermost portion of the carrierparticles. This can be desirable since this is the surface of thecatalyst system that interacts most readily with an oxidizable gas, forinstance in a burner system.

The palladium metal may be in the form of metal, an oxide, or some otheroxidized form, and may have an oxidation state of, for instance, 0, +2,or +4. In general, it is preferable that at least a portion of thepalladium be present in an oxidized form during the periods where thecatalytic burner is promoting combustion.

The palladium metal that is physically vapor deposited on the carrierparticles may, in some embodiments, have a thickness of from 0.1 nm to500 nm, from 1 nm to 400 nm, or even from 5 nm to 250 nm.

For use in a boiler, the catalytic burner is typically coupled to aninlet member having an inlet port in fluid communication with the inletopening for supplying a fuel stream from a source, and mounted withinthe boiler.

Referring now to FIG. 6, exemplary boiler 600 comprises catalytic burner605 coupled to inlet member 610 and disposed within cavity 615 inhousing 630. Inlet member 610 comprises inlet port 620 in fluidcommunication with the inlet opening 625 of catalytic burner 605 forsupplying fuel stream 670 from a source. Housing 630 has exhaust port640 and heat exchanger coils 650 for circulating a heat exchange fluid(e.g., water). Once oxidized, the fuel stream is converted to exhauststream 690 that is emitted from catalytic burner 600, passes aroundinsulated baffle 680, through heat exchange coils 650, then is exhaustedthough exhaust port 640.

The boiler may further include one or more additional components suchas, for example, a drain for condensate.

Typical fuel streams are gaseous and typically comprise an oxidizablecomponent and an oxidizing component (i.e. an oxidizer). Examples ofsuitable oxidizable components include acetylene, methane, ethane,propane, butane, pentane, and combinations thereof although otheroxidizable gaseous materials may also be used. Examples of suitableoxidizing components include air, oxygen (pure or in combination withother gas(es) such as nitrogen, carbon dioxide, and/or noble gases, forexample, as in air), or another oxidizing gas (e.g., nitrous oxide).

Catalytic burners according to the present disclosure are useful, forexample, in residential, commercial, and/or industrial boilers.

As used herein, forms of the words “comprise”, “have”, and “include” arelegally equivalent and open-ended. Therefore, additional non-recitedelements, functions, steps or limitations may be present in addition tothe recited elements, functions, steps, or limitations.

SELECT EMBODIMENTS OF THE PRESENT DISCLOSURE

In a first embodiment, the present disclosure provides a catalyticburner comprising:

-   -   a porous diffuser member;    -   a porous distributor member;    -   at least one seal extending between the porous diffuser member        and the porous distributor member, thereby defining an oxidation        chamber; and    -   catalyst media particles for catalyzing an oxidation reaction of        a fuel stream to produce heat and an exhaust stream, wherein the        catalyst media particles is disposed within the oxidation        chamber, wherein the catalyst media particles comprise carrier        particles, each carrier particle comprising refractory material,        wherein at least some of the carrier particles have thereon an        outer coating comprising an oxidation catalyst, and wherein the        catalyst media particles have a void fraction of at least 0.6.

In a second embodiment, the present disclosure provides a catalyticburner according to the first embodiment, wherein the porous diffusermember comprises a first open end, wherein the porous distributor memberis disposed at least partially inside the porous diffuser member,wherein the porous distributor member comprises a second open end.

In a third embodiment, the present disclosure provides a catalyticburner according to the first or second embodiment, further comprising afirst end cap, wherein the porous diffuser member further comprises athird open end, and wherein the first end cap covers the first open end.

In a fourth embodiment, the present disclosure provides a catalyticburner according to the third embodiment, further comprising a secondend cap, wherein:

-   -   the first end cap comprises an alignment pin inwardly extending        therefrom along a longitudinal axis of the porous distributor        member;    -   the porous distributor member further comprises a fourth open        end; and    -   the second end cap covers the fourth open end, except for a        central opening in the second end cap adapted to receive the        alignment pin.

In a fifth embodiment, the present disclosure provides a catalyticburner according to the third or fourth embodiment, wherein the porousdiffuser member comprises at least one of a cylindrical conduit or afrustoconical conduit.

In a sixth embodiment, the present disclosure provides a catalyticburner according to any one of the first to fifth embodiments, whereinthe porous diffuser member comprises a spheroidal portion.

In a seventh embodiment, the present disclosure provides a catalyticburner according to any one of the first to sixth embodiments, whereinthe outer coating is discontinuous.

In an eighth embodiment, the present disclosure provides a catalyticburner according to any one of the first to seventh embodiments, whereinthe void fraction is at least 0.65.

In a ninth embodiment, the present disclosure provides a catalyticburner according to any one of the first to eighth embodiments, whereinthe porous distributor member and the porous diffuser member areseparated from each other by an average distance of from 1 to 6millimeters.

In a tenth embodiment, the present disclosure provides a catalyticburner according to any one of the first to ninth embodiments, whereinthe catalyst media particles have an average particle diameter of from0.5 millimeter to 3 millimeters.

In an eleventh embodiment, the present disclosure provides a catalyticburner according to any one of the first to tenth embodiments, whereinthe carrier particles are saddle-shaped.

In a twelfth embodiment, the present disclosure provides a catalyticburner according to any one of the first to eleventh embodiments,wherein the carrier particles have a substantially constantcross-sectional profile along their length.

In a thirteenth embodiment, the present disclosure provides a catalyticburner according to the twelfth embodiment, wherein the oxidationcatalyst comprises palladium.

In a fourteenth embodiment, the present disclosure provides a catalyticburner according to the thirteenth embodiment, wherein the palladium isdeposited by physical vapor deposition.

In a fifteenth embodiment, the present disclosure provides a catalyticburner according to any one of the first to fourteenth embodiments,wherein the outer coating comprises an inner layer comprising ceriumoxide in contact with the carrier particle and supporting an outer layercomprising the oxidation catalyst.

In a sixteenth embodiment, the present disclosure provides a catalyticburner according to any one of the second embodiment, further comprisinga gas inlet member covering the first open end of the porous distributormember, wherein the gas inlet member comprises a gas inlet port.

In an eighteenth embodiment, the present disclosure further comprises ascreen disposed at least partially inside the porous distributor member.

In a nineteenth embodiment, the present disclosure provides a boilercomprising:

a catalytic burner according to the present disclosure disposed within acavity in a housing, wherein the catalytic burner is coupled to an inletmember comprising an inlet port for supplying a fuel stream;

a heat exchanger for circulating a heat exchange fluid disposed withinthe housing, and in fluid communication with the catalytic burner,wherein the housing further comprises an exhaust port in fluidcommunication with the inlet port through the catalytic burner.

Objects and advantages of this disclosure are further illustrated by thefollowing non-limiting examples, but the particular materials andamounts thereof recited in these examples, as well as other conditionsand details, should not be construed to unduly limit this disclosure.

EXAMPLES

Unless otherwise noted, all parts, percentages, ratios, etc. in theExamples and the rest of the specification are by weight.

Palladium Content Determination Method

Palladium content for catalyst media particles was determined usinginductively coupled plasma-optical emission spectroscopy (ICP-OES)equipment (obtained under the trade designation OPTIMA 4300DV fromPerkin Elmer, Waltham, Mass.). The catalyst media particles was analyzedagainst external calibration curves generated using acid-matchedsolution standards containing 0, 0.5, 1, and 2 parts per million (ppm)of palladium. A 0.5 ppm quality-control standard was used to monitor theaccuracy of the calibration curves during the analysis. A 0.5 ppmsolution of scandium was run in-line with the catalyst media particlesand standards to serve as an internal standard.

Catalyst media particles were measured as duplicate samples. About 100mg of each duplicate sample were weighed into respective acid-washedquartz beakers. About 2 mL of concentrated sulfuric acid were added tothe sample beakers and to two empty control beakers. The beakers werecovered with acid-washed quartz watch glasses and heated at reflux(approximately 337° C.) for two hours. Next, the beakers were partiallyuncovered to allow the excess acid to evaporate until a solution volumeof approximately 0.5 mL was obtained. About 1 mL of 30% hydrogenperoxide followed by 4 mL of aqua regia (3:1 HCl:HNO₃) were added toeach beaker, and the solutions were heated to approximately 90-100° C.for 15 minutes. About 10 mL of deionized water were then added, and thesolutions were heated gently until the remaining solid had partiallydissolved. The duplicate samples and controls were cooled,quantitatively transferred into polypropylene centrifuge tubes, dilutedto 25 mL with deionized water, and placed into the ICP-OES equipment.Palladium content is reported as weight percent of palladium based onthe total weight of the catalyst media particles.

Preparation of Catalyst Media Particles Preparation of Catalyst MediaParticles A

1.8 mm gamma-alumina spheres (Sasol LLC, Houston, Tex.) were initiallyheat treated at 1200° C. for 8 hrs (hereinafter CARRIER PARTICLES A).The void fraction of CARRIER PARTICLES A is 0.38 as calculated usingEquations 1 and 2, and ASTM C20-00 (each described hereinabove). Ananoporous support material composed of oxides of cerium and aluminum(5.25 wt %), based on a modified synthesis found in Haneda et al., Bull.Chem. Soc. Jpn. (1993), vol. 66, pp. 1279-1288, was then depositedthrough controlled solvent evaporation of metal oxide sols and metalsalt solutions onto the surface of the CARRIER PARTICLES A. Briefly,CARRIER PARTICLES A were wet (to near incipient wetness) with an acidicaqueous sol containing boehmite and cerium nitrate, and the solventremoved to produce partially dried, coated spheres, which were thencalcined at 600° C. for 4 hours to form a metal oxide (hereinafterreferred to as a “Ce/Al oxide”) coating. Based on weight gain, thetypical weight percentage of the Ce/Al oxide was about 5 weight percentof the resultant coated carrier particles.

Palladium was then deposited by physical vapor deposition (PVD) onto theCe/Al oxide coated carrier particles to various loading levels usingmagnetron sputtering and a particle agitator system. Each sample of theCe/Al oxide coated carrier particles was placed in the PVD apparatusdescribed in FIGS. 1 and 2, and especially in paragraphs [0074]-[0076]of U.S. Patent Appl. Publ. No. 2009/054230 A1 (Veeraraghavan et al.),except that blades 42 did not contain holes 44. The particle agitator 16had a blade gap of 6.3 mm. The vacuum chamber 14 was then evacuated to abackground pressure of about 5×10⁻⁵ Torr (6.6 mPa) or less, and argonsputtering gas was admitted to the chamber at a pressure of about 10mTorr (133.3 mPa). A mass flow controller with read out (obtained fromMKS Instruments, Inc., Wilmington, Mass.) was used to control the flowrate of argon in the chamber, and the argon flow rate was kept at 47standard cubic centimeters per minute (sccm). The palladium depositionprocess was then carried out by applying power to the palladium sputtertarget 32 for a pre-set period of time of 2 hours, with the particleagitator shaft 40 and blades 42 being rotated at 6 rpm. The duration ofthe palladium deposition process was 2 hours. The applied palladiumsputter target power was 110 watts.

After the palladium deposition process was completed, the vacuum chamberwas vented with air to ambient conditions, and the resultant coatedparticles (CATALYST MEDIA PARTICLES A) were removed from the PVDapparatus. The palladium content was determined according to thePALLADIUM CONTENT DETERMINATION METHOD (hereinabove) to be 0.19 weightpercent Pd.

Preparation of Catalyst Media Particles B

CATALYST MEDIA PARTICLES B was a mixture of 50% (by volume) CARRIERPARTICLES A with 50% CATALYST MEDIA PARTICLES A.

Preparation of Catalyst Media Particles C

Palladium was deposited on CARRIER PARTICLES A as in the preparation ofCATALYST MEDIA PARTICLES A, resulting in CATALYST MEDIA PARTICLES C.CATALYST MEDIA PARTICLES C does not contain a Ce/Al oxide coating. Thepalladium content was determined according to the PALLADIUM CONTENTDETERMINATION METHOD (hereinabove) to be 0.18 weight percent Pd.

Preparation of Catalyst Media Particles D

Preparation of CATALYST MEDIA PARTICLES D was a mixture of 50% (byvolume) CARRIER PARTICLES A with 50% CATALYST MEDIA PARTICLES C.

Preparation of Catalyst Media Particles E

Saddle-shaped carrier particles were created using a needle extrusionprocess with a 40 weight percent solids boehmite gel as follows.Boehmite alumina powder (4824 parts, available as “DISPERAL” from SasolNorth America Inc. Houston Tex.) was dispersed by high shear mixing asolution containing water (7087 parts) and 70% aqueous nitric acid (212parts) for 13 minutes. The resulting sol-gel was aged for at least 1hour before use.

The aged sol-gel was extruded through a 10-gauge needle crimped alongthe long axis with the end cut to about 45° to induce curling of theextruded parts. Once extruded, saddle-shaped sol-gel particles weresheared off, dried, and fired. The firing profile was a 20° C./min rampto 750° C., 18 min soak, 20° C./min ramp to 1200° C., 48 min soak, and a20° C./min cooling to 25° C.

FIG. 5 is a representative optical microscopy image of a firedsaddle-shaped alpha-alumina carrier particle. The fired saddle-shapedalpha-alumina carrier particles had an average longest dimension ofabout 2 millimeters. The void fraction of these particles is 0.61 ascalculated using Equations 1 and 2, and ASTM C20-00 (each describedhereinabove). This media is henceforth referred to as CARRIER PARTICLESB. Palladium was deposited on CARRIER PARTICLES B as in the preparationof CATALYST MEDIA PARTICLES A, resulting in a Pd content of 0.21 weightpercent, resulting in CATALYST MEDIA PARTICLES E. CATALYST MEDIAPARTICLES E does not contain the Ce/Al oxide coating.

Preparation of Catalyst Media Particles F

CATALYST MEDIA PARTICLES F was a mixture of 50% (by volume) CARRIERPARTICLES B with 50% CATALYST MEDIA PARTICLES E.

Preparation of Catalyst Media Particles G

FeCrAl metal fiber mesh, available under the trade designation BEKAERTBEKINIT 100 from Bekaert, Kortrijk, Belgium, having a thickness ofapproximately 1.5 mm and an areal density of 1,500 g/m² was heat treatedto 900° C. for 4 hours resulting in CARRIER MESH A. Ce/Al oxide wasapplied using a method analogous to those described in CATALYST MEDIAPARTICLES A resulting in a Ce/Al oxide content of 3.56 weight percent.Physical vapor deposition was used to then vapor coat CARRIER MESH Awith Pd at 250 watts for 5 min to give a Pd content of 0.06 weightpercent (relative to the total substrate mass). The PVD apparatusdescribed in FIGS. 1 and 2, and especially in paragraphs [0074]-[0076],of U.S. Patent Appl. Publ. No. 2009/054230 A1 (Veeraraghavan et al.),except that the substrate was mounted on a planar support (in place of16) that was parallel to the Pd target.

Performance Testing Pressure-Drop Testing

Pressure-drop across a bed of CARRIER PARTICLES A and B was measured ina vertically packed tube with an inner diameter of 17 mm and a bedthickness of 3 to 20 mm. The bed of particles or mesh was held in placeby mesh screens (150-160 micron opening stainless steel mesh on eitherside, with metered air entering at the bottom of the bed, travelingthrough the particle bed, and then exhausting to the atmosphere at thetop. The differential pressure was calculated by measuring the pressurejust upstream of the media and atmospheric pressure downstream. The sameapparatus was used to measure the pressure drop across the CARRIER MESHA, where the fiber mesh replaces the packed bed and has a diameter equalto the inner diameter of the test apparatus. Tables 1 and 2 (below)report the results for CARRIER PARTICLES A and B, and CARRIER MESH A.

TABLE 1 PRESSURE DROP (kPa), bed thickness = 3 mm thick FLOW PRESSUREDROP (kPa) CARRIER CARRIER RATE (slm) CARRIER MESH A PARTICLES APARTICLES B 20 0.13 0.14 0.08 40 0.38 0.43 0.24 60 0.76 0.88 0.47 801.24 1.44 0.74

TABLE 2 PRESSURE DROP (kPa) FLOW RATE bed thickness = 20 mm (slm)CARRIER PARTICLES A CARRIER PARTICLES B 20 0.69 0.25 40 2.16 0.78 60 4.51.62 80 7.48 2.7

Example 1

Boiler testing was conducted in a residential sized boiler (NTI TrinityTi150, commercially available from NY Thermal—St. John, New Brunswick,Calif.). The burner is commercially available with a FeCrAl metal fibermesh. This mesh was removed and replaced with various media for testing.Experiments were run at 30% excess air (EA) and firing rates of 15.8-158MJ/hr (15-150 kBtu/hr) (equivalent to input power densities of 544-5440MJ/hr/m² (48-480 kBtu/hr/ft²). Boiler inlet water temperature was heldconstant at 60° C., with a temperature rise through the boiler (outletminus inlet temperature) of 11° C. Gas emissions were sampled off theexhaust flue of the boiler and passed through a chiller to remove anywater vapor before being sent to a series of gas analyzers. Carbondioxide, carbon monoxide, and methane concentrations in the sampled gaswere determined by infrared absorption methods using a Horiba VIA-510gas analyzer, Irvine, Calif. Nitrogen oxide (NO_(x)) concentrations weremeasured by chemiluminescence using a Teledyne T200m NO_(x) analyzerfrom Teladyne Advanced Pollution Instrumentation, San Diego, Calif.Emission data was dilution-corrected to 0% oxygen in the sampled gas.Table 3 reports NO_(x) emissions results from these tests.

The first two sets of data compare CARRIER MESH A to CATALYST MEDIAPARTICLES G. Throughout the firing range, CATALYST MEDIA PARTICLES Gshow no significant deviation from the CARRIER MESH A. This is due tothe line of sight limitation of physical vapor deposition, resulting ina surface coating with limited penetration through the mesh thickness.This surface concentrated coverage provides inadequate contact betweenthe fuel and the catalyst and limits its effectiveness. The second twosets of data are for CATALYST MEDIA PARTICLES B and CATALYST MEDIAPARTICLES A. The packed media shows significant decreases in the NO_(x)emissions at both the low and high firing rates relative to the CARRIERMESH A and CATALYST MEDIA PARTICLES G.

TABLE 3 NO_(x) EMISSIONS (ppm by volume) CATALYST CATALYST CATALYSTMEDIA MEDIA MEDIA FIRING RATE CARRIER PARTICLES PARTICLES PARTICLESkBtu/hr kW MESH A G B A 15 4.4 5.8 4.7 3.1 1.5 30 8.8 9.3 9.3 8.0 11.650 14.7 12.8 11.7 13.7 12.8 100 29.3 24.5 24.5 15.7 14.2 150 44.0 23.422.2 14.8 14.0

Example 2

Burner testing was conducted in a custom open air burner. Methane anddry air were metered using mass flow controllers (available as AALBORGGFCS-010066 from Aalborg, Orangeburg, N.Y.) and sent through a mixingchamber containing a series of perforated discs. The premixed gas wasthen combusted in a half cylindrical burner head that mimics thegeometry of the boiler burner in Example 1 and is mounted on a flatface. The temperature of the packed bed was monitored using K-typethermocouples (Omega Engineering, Stamford, Conn.) while the emissionswere measured using methods described in Example 1. Due to theentrainment of ambient gases in the gas sampling, the measured carbondioxide and unburned methane concentrations were used to scale theNO_(x) results to the appropriate combustion products for the knowninputs. Table 4, 5 summarizes the NO_(x) emissions and packed bedtemperature data for CARRIER B, CARRIER A, CATALYST MEDIA PARTICLES D,and CATALYST MEDIA PARTICLES F. Comparing first the effect of thecatalyst on NO_(x) emissions, there is a significant drop in emissionsfor both CARRIER B and CARRIER A when the media is catalyzed. The NO_(x)emissions for the CATALYST MEDIA PARTICLES F, though, show minimalimprovements over CATALYST MEDIA PARTICLES D, despite the pressure dropdifferences shown in Table 1, 2. There is however a significantdifference in the media temperature data. For CATALYST MEDIA PARTICLESF, not only does the packed bed temperature stay below 600° C. at all ofthe firing conditions, it also maintains a stable 322-575° C. over theentire firing range rather than dropping to less than 200° C. (as seenwith CATALYST MEDIA PARTICLES D). Additionally, CATALYST MEDIA PARTICLESF do not undergo the 1200° C. temperature spike at the low fireconditions as is seen with the catalyzed spheres during initial cycling.This leads to a wider operating range where the catalyst is active yetthermally stable. This has significant implications for the catalystlifetime.

TABLE 4 NO_(x) EMISSIONS (ppm by volume) POWER POWER CATALYST CATALYSTDENSITY, DENSITY CARRIER CARRIER MEDIA MEDIA BTU/hr/ft² (kW/m²)PARTICLES B PARTICLES A PARTICLES D PARTICLES F 29000 92 20.3 11.1 notmeasured not measured 58000 183 12.7 17.5 7.7 3.9 116000 366 28.7 27.94.3 15.0 174000 549 41.9 43.4 22.7 15.6 232000 732 48.0 63.2 21.7 20.1290000 915 68.3 72.9 29.7 22.4 348000 1098 89.5 97.0 34.0 28.3 4060001281 79.8 74.3 26.1 22.8 464000 1464 90.3 87.2 32.3 30.0

TABLE 5 PACKED BED TEMPERATURE, ° C. POWER POWER CATALYST CATALYSTDENSITY DENSITY CARRIER CARRIER MEDIA MEDIA (BTU/hr/ft²) (kW/m²)PARTICLES B PARTICLES A PARTICLES D PARTICLES F 29000 92 306 286 notmeasured not measured 58000 183 256 238 641 566 116000 366 192 173 1177503 174000 549 125 101 147 457 232000 732 84 69 113 428 290000 915 64 5690 402 348000 1098 53 50 78 378 406000 1281 50 43 67 358 464000 1464 4642 61 342

Other modifications and variations to the present disclosure may bepracticed by those of ordinary skill in the art, without departing fromthe spirit and scope of the present disclosure, which is moreparticularly set forth in the appended claims. It is understood thataspects of the various embodiments may be interchanged in whole or partor combined with other aspects of the various embodiments. All citedreferences, patents, or patent applications in the above application forletters patent are herein incorporated by reference in their entirety ina consistent manner. In the event of inconsistencies or contradictionsbetween portions of the incorporated references and this application,the information in the preceding description shall control. Thepreceding description, given in order to enable one of ordinary skill inthe art to practice the claimed disclosure, is not to be construed aslimiting the scope of the disclosure, which is defined by the claims andall equivalents thereto.

1. A catalytic burner comprising: a porous diffuser member; a porousdistributor member; at least one seal extending between the porousdiffuser member and the porous distributor member, thereby defining anoxidation chamber; and catalyst media particles for catalyzing anoxidation reaction of a fuel stream to produce heat and an exhauststream, wherein the catalyst media particles is disposed within theoxidation chamber, wherein the catalyst media particles comprise carrierparticles, each carrier particle comprising refractory material, whereinat least some of the carrier particles have thereon an outer coatingcomprising an oxidation catalyst, and wherein the catalyst mediaparticles have a void fraction of at least 0.6.
 2. The catalytic burnerof claim 1, wherein the porous diffuser member comprises a first openend, wherein the porous distributor member is disposed at leastpartially inside the porous diffuser member, wherein the porousdistributor member comprises a second open end.
 3. The catalytic burnerof claim 1, further comprising a first end cap, wherein the porousdiffuser member further comprises a third open end, and wherein thefirst end cap covers the first open end.
 4. The catalytic burner ofclaim 3, further comprising a second end cap, wherein: the first end capcomprises an alignment pin inwardly extending therefrom along alongitudinal axis of the porous distributor member; the porousdistributor member further comprises a fourth open end; and the secondend cap covers the fourth open end, except for a central opening in thesecond end cap adapted to receive the alignment pin.
 5. The catalyticburner of claim 3, wherein the porous diffuser member comprises at leastone of a cylindrical conduit or a frustoconical conduit.
 6. Thecatalytic burner of claim 1, wherein the outer coating is discontinuous.7. The catalytic burner of claim 1, wherein the void fraction is atleast 0.65.
 8. The catalytic burner of claim 1, wherein the porousdistributor member and the porous diffuser member are separated fromeach other by an average distance of from 1 to 6 millimeters.
 9. Thecatalytic burner of claim 1, wherein the catalyst media particles havean average particle diameter of from 0.5 millimeter to 3 millimeters.10. The catalytic burner of claim 1, wherein the carrier particles aresaddle-shaped.
 11. The catalytic burner of claim 1, wherein the carrierparticles have a substantially constant cross-sectional profile alongtheir length.
 12. The catalytic burner of claim 1, wherein the oxidationcatalyst comprises palladium.
 13. The catalytic burner of claim 12,wherein the palladium is deposited by physical vapor deposition.
 14. Thecatalytic burner of claim 1, wherein the outer coating comprises aninner layer comprising cerium oxide in contact with the carrier particleand supporting an outer layer comprising the oxidation catalyst.